1. Field of the Invention
The present invention relates to screw-driven injection molding devices employed in the fabrication of molded items and, more particularly, to an improved injection molding ring valve assembly which utilizes a replaceable torus-shaped front seal member providing unrestricted annular mass flow during the priming phase of the injection sequence.
2. Description of the Prior Art
A variety of molding devices providing a finished molded item have been devised, including screw-driven injection molding devices. These devices typically operate by the introduction of solid, powdered material (i.e., powdered plastic) into a heated barrel wherein the material is melted. The material, while the melting process is occurring, is transported through the heated barrel by a rotating helical screw until the material reaches the injection nozzle. The pressure generated by the rotating screw further loosely packs the injector nozzle, mold runners, and mold cavities with the appropriate amount of material for the particular configuration to be molded.
The valve assembly components and the screw are then rapidly moved toward the nozzle end of the injector device. The result is extreme pressurization (often exceeding 20,000 psi) of the mold cavity ensuring the elimination of voids and stress concentrations in the finished molded item. During this phase of the injection sequence, simultaneous prevention of reverse mass flow under such high pressure back through the nozzle and into the barrel is necessary.
Preventing this reverse mass flow, nozzles are generally provided with valve assemblies that automatically interrupt mass flow through the nozzle. Commonly, ring valves having three separate components, including a valve body having an integral front valve seat, a check ring (or valve closure member) and a rear valve seat member, are used in such applications. Although generally useful for the purposes for which they are used, these three-piece ring valves do have a significant shortcoming.
This shortcoming is wear at the interface between the integral front valve seat portion of the valve body providing the seal between the valve body and the check ring. During the priming phase of the injection sequence, while the screw is rotating and transporting material toward the mold, the check ring is forced by the pressure within the barrel into abutting relation with the valve body front valve seat. This combination of rotation and pressure also imparts rotational motion to the check ring. However, as the valve body is integrally coupled with the screw mechanism, and the check ring commonly rotates at a different relative angular speed, friction and concommenent wear is the usual result. As the abutting surfaces at this interface wear, it may become necessary to replace not only the check ring, but also the valve body. The result is an expensive repair.
Attempts toward a solution of this shortcoming have included the use of a separate front valve seat member which cooperates with the valve body directing mass flow around the perimeter of the valve body into the nozzle region and also acts as a bearing surface for the valve body/check ring interface. These separate front valve seat members of previous designs where typically simple collars which mated against the valve body on one side and the check ring on the other. Mass flow was provided by machining flutes or radial cut-outs about the outer circumference of the front seal member. Thus, if wear occurred, replacement of only the less expensive front valve seat member or the check ring was necessary. The useful life of the far more expensive valve body was accordingly extended.
Unfortunately, the use of a separate front valve seat member also created its own unique set of shortcomings, including restricted mass flow through the valve assembly and the nozzle, higher unit pressures which causing faster wear at the front valve seat member/check ring interface from voids created by the flutes or radial cut-outs, uneven heating of the molding material by the front valve seat member, and the difficulty of complete cleaning caused by the crevices created by the flutes or radial cut-outs.
The rough and abrupt flow transitions present in fluted or radially cut-out front valve seat members form undesirable pressure heads that retard the necessary mass flow rates. In terms of injection device productivity, lower mass flow rates can substantially increase the injection sequence cycle time--indeed, with some fluted or radially cut-out configurations, the cycle time can be increased by more than 80%. These abrupt transitions also set up high shear stresses in plastics that can result in a lower strength of the finished molded item.
Further, the higher unit pressures existing on the remaining material forming the smaller front valve seat bearing surface accelerate valve seat wear. Moreover, the abrupt transition from the fluted or radially cut-out voids to the portions of the front valve seat where material was retained to provide a front valve seat, often at a sharp edge, causes broaching which further accelerates wear and also generates uneven "hot spots" on the front valve seat member which in turn causes uneven heating of the molding material just prior to and during the injection phase of the injection sequence.
Also, the configuration of the front valve seat members of the prior art promotes the accumulation of deposits of the melted material, which forms "chilled" regions in the material as injected into the mold and causes corrosion of the valve assembly components, with less than desirable results. This configuration also interferes with the proper cleaning of the components and the elimination of these deposits.
The improved injection molding valve of the present invention provides a new approach in reducing wear through the use of a separate front valve seat member, while at the same time allowing unrestricted annular mass flow through the valve assembly. In the present invention, the front valve seat member is configured in the shape of a torus. The torus is provided with a hub portion for concentric mounting on the shaft of the valve body, an annular outer valve seat portion for sealing engagement with the check ring and radially projecting streamlined web portions which connect the hub portion with the annular outer valve seat portion. The result is a semi-annular opening allowing unrestricted annular mass flow through the valve assembly and a continuous valve seat surface, without the discontinuities and shortcomings of flutes or radial cut-outs.